S3 is worth a watch, especially if you're going to marathon it. It's terribly uneven, yes, but it also has some of the very highs the show or superhero shows as a whole ever had. Plus it's incredibly ballsy. Not everything they did worked, but they've tried some crazy stuff I just doubt anyone will ever attempt in this genre again.
By comparision S4 feels a lot more safe. And untill break it worked. It had zero bad episodes before break. And it even managed to pull the LoT built up gracefully, which Flash failed at. Which is a shame that it's starting to stumble so badly in recent weeks. And once again, the worst element of S3, which is Felicity drama, is returning to wreck havoc in S4. Ehh..